Compressive Dewatering

Another means of reducing the amount of unwanted substances being carried

along with the pulp is liquor removal by means of mechanical pressing. The principle

of compressive dewatering is illustrated schematically in Fig. 5.13. The pulp

feed enters a device where it can be subjected to mechanical pressure (1). The

pressure then drives the filtrate (also called the pressate) out of the pulp mat (2).

Initially, the filtrate represents mainly free liquor from around the fibers. As the

pressure increases, an increasing amount of the liquor is also forced out from the

fiber voids (3).

5 Pulp Washing

Pulp feed

Filtrate

1 2

Pulp discharge

Fig. 5.13 The principle of compressive dewatering.

Unlike displacement washing, which is based on a change in liquor concentrations,

compressive dewatering diminishes the amount of unwanted substances in

the pulp discharge by reducing the volume of liquor. If sufficient time has been

allowed before pressing for intra-fiber and extra-fiber concentrations to even out,

the concentrations in the liquor entering with the pulp, in the filtrate and in the

liquor leaving with the pulp are the same.

In reality, the liquor inside the fibers often has a higher concentration than the

free liquor around the fibers due to sorption or due to a lack of diffusion time.

The measured discharge concentration will then be higher than the measured

feed concentration because highly concentrated liquor is set free from inside the

fibers only in the final phase of pressing.

5.3.4